When stereomicroscopes are used as surgical microscopes in eye surgery, for certain applications the angle between the illumination axis and observation axis of the microscope must be minimized. The result of this is that the light rays incident perpendicularly onto the eye are reflected diffusely by the retina, and the lens capsule of the eye is therefore seen by reddish transmitted light. This effect is also known as the “red reflex.” The quality of this red reflex is of critical importance, for example, in cataract extraction. In this procedure, after removal of the lens of the eye all the tissue residues must be removed from the eye. This can be achieved only if these tissue residues are presented with sufficient optical contrast.
EP 0 661 020 B1 discloses a switchable illumination device for a surgical microscope for eye surgery. The construction of this illumination system corresponds to a Köhler illumination system, such that in a first switch position the luminous-field diaphragm is imaged into the object plane, and the filament of the light source is imaged onto the retina of the eye. In at least one further switch position, the image of the filament can be shifted into the image plane of the field diaphragm. In order to ensure stereoscopic illumination, the illumination beam path is physically split using a beam splitter, in order to guide the light into the vicinity of the optical axes of the observation beam paths.
In complicated cases—for example caused by strong pigmentation of the patient's eye, a small pupil, or a high-grade cataract—the red reflex that is desirable in a cataract operation may not be as pronounced as a surgeon wishes. In such cases the red reflex can be so weakly pronounced that the eye to be operated on must be specially oriented in order to allow detection and removal of residues of the lens, which is destroyed during the operation.
The object that presents itself is therefore that of providing an illumination device for a surgical microscope in such a way that a red reflex can be efficiently made available even under the aforementioned difficult surgical conditions.